The present invention relates to a pneumatic radial tire having a carcass ply composed of metallic cords, and more particularly to an improvement of the reinforcing structure for the bead part of a radial tire for heavy loads.
In the pneumatic radial ply tire of which the carcass has a ply of metallic cords arranged radially or semi-radially at an angle of 0 to 30 degrees to a tire radial direction, the stiffness of such carcass construction is soft in comparison with the bias ply tire, and this contributes to the improvement in ride comfort of a radial tire. However, on the other hand, the amount of deformation of the tire during tire revolution is larger than that of a bias ply tire, as a result, the bead section is more prone to structural injury. Thus, there is a weak point that the bead part may be damaged in a short period of time under severe service conditions.
Heretofore, in order to solve the above mentioned technical problem characteristic in radial ply tires, many kinds of reinforced constructions of the bead part have been proposed.
One typical excellent reinforced construction of bead part is, for example, that shown in Japanese Patent No. 967452, wherein the construction, as shown in FIG. 2, is characterized in that:
a carcass ply 2 of radially arranged metallic cords, is turned up around a pair of bead cores 3 from the inside toward the outside thereof; PA0 a metallic cord reinforcing layer 4 composed of at least one layer of metallic cords being crosswise to the carcass ply cords, is disposed on the axially outside of the turned-up parts 2' of the carcass ply 2; PA0 an organic fiber cord reinforcing layer 6 composed of at least two layers of rubberized organic fiber cords, is arranged on the axially outside of the metallic cord reinforcing layer 4; PA0 the organic fiber cords of the reinforcing layer 6 are crosswise to the next layer and crosswise to the metallic cords of the metallic cord reinforcing layer 4; and PA0 an apex rubber composed of a stiffener 11 and a buffer 12, is disposed above the bead core 3 between the main part and turned-up part 2' of the carcass ply 2. PA0 a carcass ply of metallic cords is turned up around a pair of bead cores from the inside toward the outside; PA0 a metallic cord reinforcing layer is disposed on the axially outside of the turned-up parts of the carcass ply; PA0 an organic fiber cord reinforcing layer is disposed on the axially inside of the main part of the carcass, and extends to a bead part along the outside of the metallic cord reinforcing layer to cover the radially inner end of the metallic cord reinforcing layer between the carcass ply and the organic fiber cord reinforcing layer. PA0 a pair of beads each provided with a bead core; PA0 a carcass ply of metallic cords, the ends of which are turned up around the bead cores from the axially inside to outside thereof to form a carcass main part and a carcass turned up part axially inward and outward of the bead core, respectively; PA0 a metallic cord reinforcing layer turned up around the bead core to form an inside part and an outside part axially inward and outward of the bead core, respectively; PA0 an outside reinforcing layer of organic fiber cords, disposed axially outside the outside part of the metallic cord reinforcing layer; PA0 an inside reinforcing layer of organic fiber cords, disposed on the axially inside of the carcass main part; PA0 a bead apex, disposed between the carcass main part and the carcass turned up part, and extending radially outwardly beyond the radially outer end of the outside reinforcing layer; PA0 a rubber chafer disposed axially outside the outside reinforcing layer; and PA0 an inner sidewall disposed between the outside reinforcing layer and the rubber chafer.
The tire constructed as above has been favorably accepted by users as a tire which has largely improved reinforced construction of bead parts, and which can be used under severe and heavy load condition.
However, recently, there has increased the rate of using radial tires in special service conditions such as running under heavy load and high internal air pressure, and further in consequence of the remarkable improvement of highways travelling distances have been considerably increased. Accordingly, some drivers have begun to use tires under more severe conditions such as high speed and long distance running under heavy loads and high internal air pressure. As a result, bead separation failure is sometimes induced, and there has been an increased demand for heavy duty radial tires.
On the other hand, it is regarded as a sales point for steel radial tires to endure twice or thrice use by recapping the worn tire, and in order to substantially reduce the running cost, the rate of using recapped tires tends to increase. Under such circumstances, it is vigorously desired to provide a highly durable reinforced bead construction, which is vital to preserving a base tire.
Considering the bead part of radial tires as a composite construction of rubber and fibers, and taking the movement of the carcass ply in the bead part under loaded running into consideration, when the tire is inflated to a high internal pressure P as shown in FIG. 3, the carcass 2 moves upwardly as shown by an arrow 22, and the turned up part 2' of the carcass 2 moves downwardly as shown by an arrow 21. In the extreme, the carcass 2 may be pulled out from the bead core 3, which is the so-called "ply blowout" phenomenon. The metallic cord reinforcing layer 4 acts to prevent the carcass 2 from "ply blowout" because it is disposed at an angle of 50 to 70 degrees with respect to the carcass ply cords.
Furthermore when the tire is loaded, the shape of the bead part in the ground contacting part of the sidewall changes from the dotted line to the solid line as shown in FIG. 4, and the outside (TO) of the sidewall over the rim flange is subjected to deformation towards the axially outside of the tire. As a result, the cord space (CP) in the turned up part 2' of the carcass 2 is widened as shown in FIG. 5. Similarly an increase in cord spacing occurs in the main part of the carcass 2, too.
The more severe the cord space increasing phenomenon, the more the stress concentration on the rubber at the radially outer end of the turned up part 2', accordingly, a peeling phenomenon occurs earlier and grows to result in bead separation failure.
As described above, the sidewalls of the radial ply tire is soft, and it's flexure in a plane parallel to the rotary axis of the tire is very large. Thus, the amount of deformation of the tire during revolution is larger than that of the bias ply tire.
The organic cord reinforcing layer 6 in the above-mentioned Japanese Patent No. 967452, which is disposed axially outside the metallic cord reinforcing layer 4 and composed of organic cords laid at an angle of 60 to 80 degrees with respect to the carcass ply cords, alleviates the stress concentration at the end 4a thereof, but with regard to the prevention of the above mentioned cord space increasing phenomenon the reinforcement in the bead part is still insufficient.
One prior technique aimed at prevention of the cord space increasing phenomenon is, for example, that shown in UK published Patent Application GB 2150894A, wherein, as shown in FIG. 6, a hard rubber strip (HB) is disposed on the axially outside of the main part of the carcass 2. However this means using rubber is still insufficient in terms of increasing circumferential stiffness of the carcass ply 2.
Further, another prior technique aimed at prevention of the cord space increasing phenomenon is that shown in Japanese Patent Application JP A 60-56608, wherein the construction is characterized in that:
Arranging a large number of piles of cords in the bead part is a common way to reinforce the bead part, but a simple increase in the cord ply number results in an increased number of the ends thereof and in the increased stiffness of the cord ends. Accordingly, stress concentration at the cord ends and interlayer stress between the adjacent cord plies are apt to increase, and peeling damage between the cord ends and the rubber and separation between the adjacent plies is sometimes induced. Therefore, the durability of the tire is decreased contrary to desired objectives and the bead parts rather tend to suffer from bead damage.